From a lifespan perspective (NIAAA, 2007), emerging adulthood is a developmental stage characterized by greater social autonomy and new freedoms that can place young adults at higher risk for alcohol- and other drug-related problems. Indeed, epidemiological data reveal that it is during this transitional period that the highest rates of alcohol and other drug use disorders occur (Grant, Dawson et al, 2004;SAMHSA, 2007). Furthermore, clinical data indicate that co-occurring mental health conditions are also most prevalent among this 18-25 year old cohort (Chan, Dennis et al, 2008). Providing early, effective treatment can shorten the duration and impact of these disorders yet, compared to adolescents and older adults, little is known about how young adults respond to treatment or achieve salutary changes. Furthermore, while more than three quarters of US treatment programs espouse a 12-step orientation (Roman et al, 2004) it remains one of the least evaluated (Miller &Willbourne, 2003). Also noteworthy from a treatment evaluation perspective, is that most clinical trials that compare psychosocial interventions for substance use disorders (SUDs) observe broadly equivalent outcomes among conditions despite marked differences between interventions in content and conceptualization of the disorders'causes and maintaining conditions (e.g., Dennis, Godley et al, 2004;Morgenstern &McKay, 2007;Project MATCH Research Group, 1997). This has led to a renewed focus on uncovering essential mechanisms by which individuals change their alcohol/drug use and how the relative influence of such mechanisms may change over the course of recovery in order to develop stage-appropriate interventions that target key recovery processes (Finney, 2007;Kazdin &Nock, 2003;Nock, 2007). To examine these issues, the current study employs a large clinical dataset of young adults treated in 12-step- oriented residential treatment (N = 304, M age = 20.4 [range = 18-25], 27% female;97% White) and assessed at multiple points to elucidate some of the critical mechanisms and moderators of 12-step-oriented treatment and continuing care effects. Greater knowledge of 12-step treatment, related mutual-help group participation, and the mechanisms and moderators of change among emerging adults will provide important developmentally-specific clinical information for this highest risk cohort, and inform our broader knowledge regarding mechanisms of change that will lead to the development and testing of more targeted and efficient interventions. PUBLIC HEALTH RELEVANCE: Rates of alcohol abuse and dependence peak during emerging adulthood (18-25 years old), yet we know little about the effectiveness of treatment during this highest risk developmental period. Additionally, little is known about how and why treatment works, which has limited the effectiveness and efficiency of our interventions. This study will elucidate the mechanisms through which treated young adults achieve and maintain recovery and also investigate the moderators of treatment response. Greater clarification of such mechanisms and moderators will lead to the development of more targeted and effective interventions.